1. Field of the Invention
The present invention relates to a surface acoustic wave device. More specifically, the present invention relates to a surface acoustic wave device having a so-called divided type interdigital electrode wherein at least one of two interdigital electrodes is divided into at least two in the propagating direction of the surface acoustic wave and the divided interdigital electrodes are electrically connected in series.
2. Description of the Prior Art
Generally, an interdigital electrode used as a transducer of a surface acoustic wave device comprises a set of comb shaped electrodes having a number of electrode fingers and which are interdigitated each other. The static capacitance of the transducer is determined as the sum of the capacitances between the adjacent opposing electrode fingers. The capacitance of the transducer becomes relatively large when the dielectric constant of a piezoelectric substrate of the surface acoustic wave device is large. Accordingly, in assembling a surface acoustic wave device in an electric circuit, a difficulty could arise in attaining the impedance matching of the same with a preceding stage circuit and/or a succeeding stage circuit. In order to eliminate such difficulty, an approach has been proposed wherein the interdigital electrode is divided into a plurality of portion electrodes in the propagating direction of the surface acoustic wave and the plurality of portion electrodes are electrically connected in series, so that the total capacitance may be decreased. In employing the above described approach, in case of a surface acoustic wave of a normal electrode pattern type, for example, the electrode pattern is selected such that the electrode pattern may be symmetrical with respect to left and right, including the point symmetry, for the purpose of preventing degradation of the frequency characteristic due to the above described division. According to a symmetrical arrangement of the electrode pattern in case of a surface acoustic wave device having a normal electrode pattern, it is supposed that a sufficiently good frequency characteristic is attained insofar as attention is given to the electrode pattern, unless stray capacitances are considered.
A surface acoustic wave device having a divided type interdigital electrode is disclosed in, for example, U.S. Pat. No. 3,600,710 issued Aug. 17, 1971 to Robert Alder; British Pat. No. 1,362,238 (complete specification published July 31, 1974); U.S. Pat. No. 4,143,343 issued Mar. 6, 1979 assigned to the same assignee as the present invention; and others.
FIG. 1 is a view showing one example of a surface acoustic wave device showing the background of the invention. Referring to FIG. 1, a surface acoustic wave device comprises a set of interdigital electrodes 2 and 3 formed on a piezoelectric substrate 1. The interdigital electrodes 2 and 3 constitute an input (or an output) transducer and an output (or an input) transducer, respectively. The set of interdigital electrodes 2 and 3 are formed spaced apart a predetermined distance and a shield electrode 4 is formed as necessary on the substrate 1 between the electrodes 2 and 3. One interdigital electrode 2 out of the set of interdigital electrodes 2 and 3 is divided into a plurality of portion electrodes 21 and 22 (two in the embodiment shown), spaced apart in the propagating direction of the surface acoustic wave. The divided portion electrodes 21 and 22 are electrically connected in series. The portion electrode 21 comprises two comb shaped electrodes. One comb shaped electrode out of these two comb shaped electrodes comprises a plurality of electrode fingers 211 and a connection electrode 212 for commonly connecting one end of each of these electrode fingers 211. The other comb shaped electrode comprises a plurality of electrode fingers 231 and a connection electrode 232 for commonly connecting one end of each of these electrode fingers 231. These electrode fingers 211 and 231 are interdigitated forming one portion interdigital electrode (referred to as "portion electrode" hereinafter). The portion electrode 22 comprises two comb shaped electrodes. One comb shaped electrode out of these two comb shaped electrodes comprises a plurality of electrode fingers 221, and a connection electrode 222 for commonly connecting one end of each of these electrode fingers 221. The other comb shaped electrode comprises a plurality of electrode fingers 231 and a connection electrode 232 for common connecting one end of each of these electrode fingers 231. These electrode fingers 221 and 231 are interdigitated to form one portion interdigital electrode. Lead electrodes 51, 52, 53 and 54 are further formed on the substrate 1. Although not shown, these lead electrodes 51 to 54 are connected to lead terminals. Since a general operation of such surface acoustic wave device is well-known to those skilled in the art, it is not believed necessary to describe the same in detail.
Such a surface acoustic wave device has been used as a filter constituting a video intermediate frequency circuit of a television receiver, for example. FIG. 2 is a graph showing one example of a filter characteristic of such surface acoustic wave device. The filter has been designed such that the central frequency is approximately 36.5 MHz and attenuation poles are exhibited in the vicinity of a lower frequency of approximately 32 MHz and a higher frequency of approximately 40.4 MHz.
On the other hand, in using such a surface acoustic wave device having a filter characteristic as shown in FIG. 2, for example, conventionally it has been believed that most preferably the static capacitances and thus the electrode patterns (in case of a normal electrode pattern type) of the portion electrodes 21 and 22 shown in FIG. 1 should be the same and accordingly such surface acoustic wave device has been conventionally designed in such belief. However, when the other interdigital electrode, the shield electrode and so on are also formed on the substrate, as shown in FIG. 1, and a surface acoustic wave device is completed by using lead terminals, not shown, a package, not shown, and so on and then such surface acoustic wave device is installed in an actual video intermediate frequency circuit of a television receiver, for example, i.e. when a surface acoustic wave device is actually installed, a phenomenon was observed in which the attenuation at the attenuation poles becomes small, as shown by the curve A in FIG. 2. Nevertheless, conventionally it has not been believed that the reason why the characteristic of the attenuation poles is degraded when a surface acoustic wave device is actually installed is that such degradation is related to the static capacitance of the electrode but rather that it is related to some other cause. For example, in designing an electrode pattern of a surface acoustic wave device, generally a Fourier transformation has been employed, whereby the electrode pattern is determined. It was believed that a discrepancy between the Fourier transformation employed in designing such electrode pattern and the actual design was one of such causes. Conventionally it was also believed that the accuracy in the actual manufacture of a piezoelectric material and an electrode pattern was another cause. Accordingly, conventionally, even when degradation occurred in characteristic of the attenuation poles as shown by the curve A in FIG. 2, for example, consideration was given only to accuracy in designing an electrode pattern and manufacturing a surface acoustic wave device and various improvements were made thereon. Nevertheless, in spite of such an attempt to improve the characteristic in attenuation poles, a surface acoustic wave device of a filter characteristic having a satisfactory degree of attenuation at the attenuation poles was not obtained.
Under these circumstances, the inventor turned his attention to the static capacitance of the divided portion electrodes based on his doubt that degradation of the attenuation at the attenuation poles is related to the static capacitance of the electrode and made one calculation of interest. According to the calculation, in a chip not packaged and installed in a circuit, i.e. in a chip including only the interdigital electrode 2, i.e. the portion electrodes 21 and 22 on the substrate 1, as shown in FIG. 1 the static capacitances of the portion electrodes 21 and 22 were intentionally changed, whereupon the frequency characteristic was calculated. As a result, the characteristic as shown in FIG. 3 was obtained. The curves A, B and C in FIG. 3 all show the frequency characteristics of the interdigital electrode 2 divided into the portion electrodes 21 and 22. More specifically, the curve A of FIG. 3 shows a calculation result obtained when the static capacitances of the two portion electrodes 21 and 22 are made different by 20% from each other, the curve B shows a calculation result obtained from the static capacitances of the two portion electrodes 21 and 22 are made different by 10% from each other, and the curve C shows a calculation result obtained when the static capacitances of the two portion electrodes 21 and 22 are the same. From the calculation result shown in FIG. 3, it is hypothesized that as far as the attenuation poles of the filter characteristic are concerned it is most preferred to select the static capacitances of the two portion electrodes 21 and 22 to be the same. Accordingly, in case of a normal electrode pattern type, the conventional belief that the electrode patterns of the respective portion electrodes 21 and 22 should be disposed in a symmetrical manner, including the point symmetry, was justified in making the respective static capacitances the same. Nevertheless, the above described degradation of the attenuation pole characteristic occurs in installing a surface acoustic wave device in an electric circuit. Therefore, the inventor considered the cause of degradation by paying his attention to this point and reached the following conclusion. More specifically, the inventor noticed that even when the electrode patterns of the divided portion electrodes 21 and 22 shown in FIG. 1, for example, are formed in a symmetrical manner, thereby to make the static capacitances of the portion electrodes to each other, the equal capacitances are equal only when only the portion electrodes 21 and 22 are formed on the substrate 1. Then the inventor hypothesized that imbalance of the static capacitances could have occurred due to the formation of the other interdigital electrode 3, the shield electrode 4, the lead electrodes 51 to 54, and further formation of a rear surface electrode, as necessary, and so on.